1. Field of the Invention
The present invention relates to a variable-displacement vane pump, and more particularly, to a variable-displacement vane pump which is suitable for supplying an operation fluid to an automotive power steering system.
2. Discussion of the Prior Art
A hydraulic pump is used for a power steering system in a vehicle. The amount of the operation fluid which is discharged from the pump is preset so that the pump can support a steering operation sufficiently even under low speed driving, during which the rotational speed of an engine is low. Further, the hydraulic pump discharges the operation fluid in proportion to the rotational speed of the engine. Therefore, the amount of the operation fluid discharged from the pump becomes excessive under high speed driving, during which the rotational speed of the engine is generally high.
To solve the above-mentioned problem, a flow control valve has been generally adopted in the power steering system whereby part of the operation fluid discharged from the pump is returned to the pump through a bypass passage without transmitted to a power assistant mechanism of the power steering system. The high pressurized fluid discharged from the hydraulic pump is led into the flow control valve. An excessive portion of the operation fluid is discharged to the bypass passage to be returned to intake ports formed in the pump. Consequently, a large amount of energy is expended in proportion to the rotational speed of the engine when the engine rotates at a high speed. Namely, the energy is lost under high speed driving, during which little steering support is needed, resulting in an increase of fuel consumption rate of the vehicle.
A switching valve has been conventionally used in the pump in order to reduce the energy loss, as described in Japanese Laid-open Patent Publication No. 60-256579. This is a variable-displacement vane pump which consists of a pump part and a switching valve 1, as shown in FIG. 1. The pump part is mainly comprised of a housing 2, a rotor 6, vanes 7, a cam ring 5, side plates 3 and 4, intake ports 41 and 41', and exhaust ports 31 and 31'. The vane pump is further provided with a discharge pressure chamber 27 which is connected to the exhaust port 31'.
In the switching valve 1 is formed a cylindrical chamber 13 in which a spool 11 and a spring 15 are received. At one end of the spool 11, a pressure chamber 14 is formed to be communicated with the discharge pressure chamber 27. At the other end thereof, a spring chamber 16 is formed to receive the spring 15. The switching valve 1 is further provided with an inlet port 18 and an outlet port 19. Operation fluid is sucked from the inlet port 18 to be led into the intake ports 41 and 41' through the spring chamber 16. The outlet port 19 is connected to the power assistant mechanism of a power steering system via a flow control valve (not shown). The outlet port 19 is connected to the discharge pressure chamber 27 via the pressure chamber 14 formed in the switching valve 1.
With this configuration, when the power assistant mechanism does not operate, the pressure at the outlet port 19 is low. In such state, the difference in pressure between the pressure chamber 14 and the spring chamber 16 is small. The spool 11 is thus placed to the left by the force of the spring 15. As a result, the second intake port 41' is separated from the inlet port 18 by the spool 11, and is connected to the discharge pressure chamber 27 via the cylindrical chamber 13. Under this condition, part of the operation fluid discharged from the exhaust ports 31 and 31' is returned to the second intake port 41' through the discharge pressure chamber 27 and the cylindrical chamber 13, as illustrated by the broken-line arrow in FIG. 1. Namely, the operation fluid is only circulated between the second exhaust port 31' and the second intake port 41'. The pumping action does not occur in such condition. Consequently, the amount of the operation fluid discharged from the pump does not increase and the energy loss during the pumping action is lowered.
The above-mentioned vane pump has a problem that the energy loss cannot be decreased sufficiently, because the operation fluid only circulated between the second exhaust port 31' and the second intake port 41' is pressurized fluid having a high pressure. Namely, when the circulated fluid has a high pressure, the energy loss produced during the circulation cannot be ignored.
To overcome this problem, there has been proposed another variable-displacement pump as disclosed in the Japanese Laid-open Patent Publication No. 61-119472. In this vane pump, the vane pump is divided into a pair of pump portions, each of which has an intake port and an exhaust port. The vane pump is further provided with a switching valve which connects the intake port and exhaust port of a particular pump portion to stop its pumping action when the load pressure is low. Since the operation fluid circulated between the exhaust and intake ports of the particular pump portion is non-pressurized fluid having a low pressure, the energy loss is lowered as compared with the conventional vane pump disclosed in Japanese Laid-open Patent Publication No. 60-256579.
By the way, the vane pump also has a flow control valve, and uses a so-called supercharge effect for efficiently sucking the operation fluid to the pump by using the energy of the operation fluid returned from the flow control valve. However, where the function of the particular pump portion is stopped, the amount of the operation fluid which is returned from the flow control valve is reduced, thereby lowering its supercharge effect. The decrease of the supercharge effect may cause cavitation in the pump chambers.
There is another problem in such vane pump. Though part of the operation fluid is only circulated between one of the exhaust ports and the intake ports, the energy loss during the circulation operation isn't small to be ignored. The reason will be described hereinafter.
When the vane 7 is rotated, a thrust force toward a cam ring 5 acts on the vane 7, as shown in FIG. 2. When a particular pump chamber 56 is located in a particular pump portion which stops it pumping action, operation fluid having low pressure is circulated through the pump chamber 56. In such state, the pressure Ps at the outer end of the vane 7 is low while the pressure Pa at the bottom 61 of a slit 6a receiving the vane 7 is the same as that of the operation fluid which is discharged from the other acting pump portion and the pressure of which is therefore high. Accordingly, the vane 7 is pressed to the inner periphery surface of the cam ring 5. Since the vane pump is operated under this condition, it is impossible to reduce the energy loss sufficiently.